Mutant rice is disclosed that is (1) resistant/tolerant to both HPPD and ACCase inhibiting herbicides; or (2) resistant/tolerant only to 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibiting herbicides. Methods of weed control are disclosed using rice with these herbicide resistant/tolerant crops in fields. Methods to produce herbicide resistant/tolerant rice are also disclosed.
Value of Rice Crops
Rice is an ancient agricultural crop and today is one of the principal food crops of the world. There are two cultivated species of rice: Oryza sativa L., the Asian rice, and Oryza glaberrima Steud., the African rice. The Asian species constitutes virtually all of the world's cultivated rice and is the species grown in the United States. Three major rice producing regions exist in the United States: the Mississippi Delta (Arkansas, Mississippi, northeast Louisiana, southeast Missouri), the Gulf Coast (southwest Louisiana, southeast Texas), and the Central Valley of California. Other countries, in particular in South America and the East, are major rice producers.
Rice is one of the few crops that can be grown in a shallow flood as it has a unique structure allowing gas exchange through the stems between the roots and the atmosphere. Growth in a shallow flood results in the best yields and is the reason that rice is usually gown in heavy clay soils, or soils with an impermeable hard pan layer just below the soil surface. These soil types are usually either not suitable for other crops or at best, the crops yield poorly.
The constant improvement of rice is imperative to provide necessary nutrition for a growing world population. A large portion of the world population consumes rice as their primary source of nutrition and crops must thrive in various environmental conditions including competing with weeds and attacks by unfavorable agents. Rice improvement is carried out through conventional breeding practices and also by recombinant genetic techniques. Though appearing straightforward to those outside this discipline, crop improvement requires keen scientific and artistic skill and results are generally unpredictable.
Although specific breeding objectives vary somewhat in the different rice producing regions of the world, increasing yield is a primary objective in all programs.
Plant breeding begins with the analysis and definition of strengths and weaknesses of cultivars in existence, followed by the establishment of program goals, to improve areas of weakness to produce new cultivars. Specific breeding objectives include combining in a single cultivar an improved combination of desirable traits from the parental sources. Desirable traits may be introduced due to spontaneous or induced mutations. Desirable traits include higher yield, resistance to environmental stress, diseases and insects, better stems and roots, tolerance to low temperatures, better agronomic characteristics, nutritional value and grain quality.
For example, the breeder initially selects and crosses two or more parental lines, followed by selection for desired traits among the many new genetic combinations. The breeder can theoretically generate billions of new and different genetic combinations via crossing. Breeding by using crossing and selfing, does not imply direct control at the cellular level. However, that type of control may be achieved in part using recombinant genetic techniques.
Pedigree breeding is used commonly for the improvement of self-pollinating crops such as rice. For example, two parents which possess favorable, complementary traits are crossed to produce an F1 generation. One or both parents may themselves represent an F1 from a previous cross. Subsequently a segregating population is produced, by growing the seeds resulting from selfing one or several F1s if the two parents are pure lines, or by directly growing the seed resulting from the initial cross if at least one of the parents is an F1. Selection of the best individual genomes may begin in the first segregating population or F2; then, beginning in the F3, the best individuals in the best families are selected. “Best” is defined according to the goals of a particular breeding program e.g., to increase yield, resist diseases. Overall a multifactorial approach is used to define “best” because of genetic interactions. A desirable gene in one genetic background may differ in a different background. In addition, introduction of the gene may disrupt other favorable genetic characteristics. Replicated testing of families can begin in the F4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F6 and F7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new parental lines.
Backcross breeding has been used to transfer genes for a highly heritable trait into a desirable homozygous cultivar or inbred line which is the recurrent parent. The source of the trait to be transferred is called the donor parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The process is used to recover all of the beneficial characteristics of the recurrent parent with the addition of the new trait provided by the donor parent.
Promising advanced breeding lines are thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for at least three or more years. The best lines are candidates for new commercial varieties or parents of hybrids; those still deficient in a few traits may be used as parents to produce new populations for further selection.
These processes, which lead to the final step of marketing and distribution, usually take from 8 to 12 years from the time the first cross is made and may rely on the development of improved breeding lines as precursors. Therefore, development of new cultivars is not only a time-consuming process, but requires precise forward planning, efficient use of resources, and a minimum of changes in direction. The results include novel genetic combinations not found in nature.
Some improvement of rice through breeding may be restricted to the natural genetic variation in rice and hybridizing species, such as wild rice. The introduction of new variation in a breeding program is usually through the crossing program as described, such as pedigree or backcross breeding. However, occasionally natural mutations are found that result in the introduction of new traits such as disease resistance or height changes. Breeders have also developed new traits by inducing mutations (small changes in the DNA sequence) into a rice genome. Some of these mutations or combination of genes are not found in nature. Commonly, EMS or sodium azide plus MNU are used as mutagenic agents. These chemicals randomly induce single base changes in DNA, usually of G and C changed to A and T. Overall effects are unpredictable. Most of these changes have no effect on the crop as they fall either outside the gene coding regions or don't change the amino acid sequence of the gene product. However, some produce new traits or incorporate new DNA changes into previous lines.
The breeder has no direct control of mutation sites in the DNA sequence. The identification of useful changes is due to the random possibility that an effective mutation will be induced and that the breeder will recognize the phenotypic effects of the change and will be able to select rice having that mutation. Seeds are treated with the mutagenic chemical and immediately planted to grow and produce M2 seed. The M2 seed will carry numerous new variations; therefore, no two experiments will produce the same combinations. Among these variations new traits previously not existing in rice and unavailable for selection by a plant breeder may be found and used for rice improvement.
To find new traits the breeder must use efficient and strategic selection strategies as the process is completely random and has an extremely low frequency of useful new combinations. Among thousands of induced new genetic variants there may be only one with a desirable new trait. An optimal selection system will screen through thousands of new variants and allow detection of a few or even a single plant that might carry a new trait. After identifying or finding a possible new trait the breeder must develop a new cultivar by pedigree or backcross breeding and extensive testing to verify the new trait and cultivar exhibits stable and heritable value to rice producers.
Using recombinant genetic techniques, nucleic acid molecules with mutations that encode improved characteristics in rice, may be introduced into rice with commercially suitable genomes. After a mutation is identified by whatever course, it may be transferred into rice by recombinant techniques.
Applications of Herbicide Resistance Patents in Rice
Weeds and other competitors for resources in crop fields compete for resources and greatly reduce the yield and quality of the crop. Weeds have been controlled in crops through the application of selective herbicides that kill the weeds, but do not harm the crop. Usually selectivity of the herbicides is based on biochemical variations or differences between the crop and the weeds. Some herbicides are non-selective, meaning they kill all or almost all plants. Non-selective or broad spectrum herbicides can be used in crops if new genes are inserted that express specific proteins that convey tolerance or resistance to the herbicide. Resistance to herbicides has also been achieved in crops through genetic mutations that alter proteins and biochemical processes. These mutations may arise in nature, but mostly they have been induced in crops or in vitro in tissue cultures or by inducing mutations in vivo. Unfortunately in some instances, especially with repeated use of a particular herbicide, weeds have developed resistance through the unintended selection of natural mutations that provide resistance. When weeds become resistant to a particular herbicide, that herbicide is no longer useful for weed control. The development of resistance in weeds is best delayed through alternating the use of different modes of action to control weeds, interrupting development of resistant weeds.
Rice production is plagued by broad leaf plants and a particularly hard to control weed called red rice. One difficulty arises because red rice is so genetically similar to cultivated rice (they occasionally cross pollinate) that there are no selective herbicides available that target red rice, yet do not harm the cultivated rice. Control is currently provided in commercial rice production through the development of mutations found in rice that render rice resistant to broad spectrum herbicides e.g. imidazolinone and sulfonylurea herbicides. Rice resistant to herbicides that inhibit other deleterious plants, such as broad leaf plants, are needed.
Finding new mutations in rice that makes it resistant to a variety of herbicides, and to combinations of herbicides with alternative modes of action, would greatly benefit rice production. Obtaining and incorporating genes for herbicide resistance into rice genomes with additional favorable characteristics and alternative resistances is challenging, unpredictable, time consuming and expensive, but necessary to meet the world's increasing food needs.